1. Field of the Invention
The invention is concerned with a process for drying of gas filling devices for fuel tanks and with a device for drying the gas comprising the gaseous volume in a fuel tank, as well as a process for drying of gas.
2. Description of the Related Art
In fuel tanks, coincidentally with the withdrawal of fluids, the volume of the withdrawn fuel is replaced by gas, and gas tanks are conventionally ventilated by ambient air. The in-flowing external air always contains an amount of water as vapor component which, depending upon the degree of saturation, can in the course of variations of the ambient temperature achieve a relative humidity of 100%, and can condense.
This problem is particularly serious, for example, in the operation of airplanes, wherein a condensation of water vapor occurs in the area of the tank during flying as a consequence of the enormous temperature variations during flying. The enriched water component leads to reduction of the exergetic effectiveness during combustion of the airplane fuel (kerosene) and to corrosion of the inside of the tank. As counter measures, various coatings for the tank inner area have been proposed for corrosion protection, which however, with respect to construction, have a high expense, in particular in joints. In the employment of CFK (carbon fiber composite), the effect of moisture can even lead to a reduction of the material quality.
In order to solve principally the problem of water contamination of the airplane fuel, systems exist, which pump water out of the fuel tank after the condensation thereof.
Until now, a system of this type is known for example from reference EP 0 278 755 A2, which in the airplane tank in pockets enriched water via a pumping mechanism admixes into the fuel stream. This water/fuel mixture is, via a propulsion pump, advanced to the propulsion system and in this manner the water component is eliminated along with the combustion process. These systems essentially serve specifically to withdraw the condensed water out of the tank and do not prevent the collection of water in the overall tank volume.
A more sensible arrangement or solution would be on the one hand to reduce or suppress the water vapor content of the incoming air by a suitable measure and, if necessary, to supplementally dry the gaseous component situated in the tank.
In the technical realm, the cold process or the over-pressurization of gases for drying of air are known. For technical reasons, these processes are however exclusively realized for drying of gases in stationary ground operations.
The cold process and the over-pressurization are already technically established technologies, which however above all for reasons of high apparatus complexity and the therefrom resulting weight problem certainly cannot contribute to the solving of the problem of drying of air in airplanes.
The cooling process represents the oldest drying technology for air drying. Therein one cools the compressed air using a cooling machine to the value below the saturation temperature. The lower the pressure dew point is to be, the greater the number of parallel coolers necessary therefore. Should for example a pressure dew point of below 2xc2x0 C. be achieved, then parallel operation of two coolers is necessary, since in temperatures of below 0xc2x0 the cooling surfaces can ice-over and subsequently must be thawed out. For the continuous operation, this means a periodic mode of operation of the individual dryers. The air saturated at 2xc2x0 C. is subsequently counter-current warmed with the incoming air so that a saturation of approximately 30% can be achieved at environmental temperature.
As further, technically already realized processes, there is the over-pressurization process, in which the air is densified by increasing pressure. Thereby, the partial pressure of the water vapor increases, until the saturation vapor pressure is achieved and condensation of the water vapor begins. Subsequently, the air is again cooled off to operating temperature, whereby a part of the water vapor condenses. The de-pressurization of the compressed air to the operating pressure brings about a further pressure dew point reduction. Since the compression of great gas volume streams is very cost intensive, the described drying methodology is suitable only for small volume flows. Analogously to the drying process using cooling machines, the technical equipment necessary is extensive, from which the employment in aircraft operation, above all for reasons of weight, again appears unlikely.
All until now known methods for gas drying lead to complex technical solutions, which have as a consequence a high weight. In particular in the transport area and in particular in the air and space these have provided until now no solution to the reduction of water component in the fuel tank.
The invention is thus concerned with the task, of providing a device, with which the condensation of water vapor in fuel tanks can be prevented as well as a process associated therewith.
The invention includes a device for drying air in ventilation devices for fuel tanks, in which in the air flow of the ventilation device vapor, permeable membranes are provided in such a manner, that the air only flows over or via one selected or predetermined surface of the membrane (here characterized or named the front side).
In the same way, this device can be employed for drying of gas in the gas volume in the fuel tank, wherein at the fuel tank a withdrawal and a return system for production of a gas flow is provided, and in the gas flow vapor permeable membranes are provided in such a manner that the air flows over the front side of the membranes.
The vapor permeable membranes are constructed as composite membrane systems, comprised of a carrier membrane for mechanical stabilization and a separation active layer of polymers, preferably of cellulose ether, cellulose sulfates, or polyvinyl alcohols.
On the back side of the vapor permeable membrane, there is provided in certain cases a further layering or bed or packing of sorbents for storage of the water component.
The vapor permeable membranes are constructed as plates, compact hollow fibers, or wrapped modules. The inventive devices are particularly suitable for employment in fuel tanks for air and space travel.
A further device for drying of air in ventilation devices for fuel tanks is a flow-throughable module provided in an air flow of the ventilation device, with sorbents for separation of the water component.
Likewise, this device can be employed for drying of gas, of the gas volume in the fuel tank, wherein also then on the fuel tank a system for achievement of a gas flow is employed and a module with sorbents for separation of the water components is provided through which the gas stream flows.
The module is constructed as a cassette with a gas permeable jacketing and sorbent packing.
The sorbents are comprised of absorbents, for example silica gel, activated aluminum oxide, zeolite. The zeolites exhibit porosities between 5 and 15 nanometers. Finer porosities require too high a pressure loss of the absorber and provide too small a capacity for receiving of water. In zeolites with greater pore diameters, too many kerosene components penetrate, which results at least partially in rendering the material hydrophobic. A pore diameter of approximately 10 nanometers has been found to be particularly suitable.
The sorbents can also be comprised of solid absorbent, for example the so-called super absorbent cellulose compounds, or lithium chloride, calcium chloride, phosphorpentoxide, or a fluid absorbent, for example lithium chloride solutions, sulfuric acid, glycols.
In the process for drying of air in the ventilation device for fuel tanks using vapor permeable membranes, the air flow of the ventilation system is conducted or conveyed over the vapor permeable membrane, wherein the water component to be separated permeates through the membranes, accumulates in the permeate, and in certain cases is bound, and the dried air in the retentate is directed back to the tank. In the same manner, in place of membranes it is also possible to employ cassettes with absorbent fillings flowed through by the air stream, in which the water component to be separated is bound using adsorption or also absorption.
In the process for gas drying in fuel tanks by means of vapor permeable membranes, gases are withdrawn from the tank, and this gas stream is conducted over the vapor permeable membranes. The water component to be separated permeates through the membranes and is accumulated in the permeate and, in certain cases, is bound. The dried gas in the retentate is directed back to the tank. Likewise there can be employed in place of the membranes again cassettes with sorbents flowed through by the air stream.
A particular advantage of the invention is comprised in the weight reduction in comparison to the technical solutions which can be found in the state of the art. Therewith, the membrane and sorption processes, due to their low apparatus complexity and costs, represent an ideal problem solution for the drying of air particularly in the transportation industry and in particular in air and space travel. The result of investigations has shown that, for example, in an airplane tank, the water component can accumulate up to 1 Vol. % in the kerosene and thus the drying of air with the objective of the prevention of water vapor accumulation and condensation in the tank represents a substantial factor in weight reduction.